Microbial density and substrate identity interact to determine the stabilization efficiency of microbial-derived, mineral-associated organic matter

Dr Noah Sokol1,2, Dr. Mark  Bradford2, Dr.  Jon Sanderman3, Dr. Jessica Gutknecht4, Dr.  Jeff Baldock5

1Lawrence Livermore National Laboratory, Livermore, United States, 2Yale University, New Haven, USA, 3Woods Hole Research Center, Woods Hole, USA, 4University of Minnesota, Minneapolis, USA, 5CSIRO, Glen Osmond, Australia

A substantial proportion of persistent, mineral-associated organic matter (MAOM) is microbially-derived. Despite the importance of this stable carbon pool for understanding and modeling the terrestrial carbon cycle, the controls on microbial formation of MAOM remain unclear. One key but under-explored control is the density of soil microbes where a carbon substrate enters the mineral soil, as a higher microbial density should increase the probability that an assimilable substrate undergoes microbial uptake and transformation prior to mineral-stabilization, whereas a lower density will decrease that probability, and increase the chance that a substrate directly encounters a mineral surface.

To understand the interacting role of microbial density and substrate identity on the efficiency MAOM formation, we directly manipulated microbial density in laboratory soil microcosms, while standardizing for the amount of added carbon substrate. We repeatedly inserted fixed quantities of four different ¹³C-labeled substrates (vanillin, phenol, glucose, and acetic acid) into microcosms with varying microbial densities. We measured uptake of these substrates into different microbial taxa (¹³C-PLFA), as well the amount of ¹³C-MAOM, and the amount of stable ¹³C-MAOM. We also analyzed the composition of the ¹³C-MAOM to determine the extent of microbial transformation of added substrates through ¹³C-NMR.

We found a significant interaction between substrate type and microbial density (F=4.4, p=0.008), as well as a significant interaction between mineral type (illite vs. allophane) and substrate type (F=3.0, p=0.04). Generally, for the less reactive mineral type ‘illite’, we found positive relationships (p<0.05) between microbial density and stable ¹³C-MAOM formation, and no significant relationship in the more reactive mineral ‘allophane’. Overall, we found that microbial formation of stable MAOM increased with microbial density, and with carbon substrates metabolized with higher carbon use efficiency (e.g. glucose versus phenol), but also that the magnitude of this relationship was modulated by the dominant mineral type.

Biography: Noah Sokol is a Postdoctoral Researcher at Lawrence Livermore National Lab, who recently completed his PhD at Yale University.

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